Secrecy communication system



United States Patent SECRECY COMMUNICATION SYSTEM Walter S. Druz,Bensenville, Ill., assignor to Zenith Radio Corporation, a corporationof Delaware Application July 26, 1956, Serial No. 600,196

12 'Claims. ((11. 178-51) This invention pertains to secrecycommunication systems 1n which an intelligence signal is transmitted incoded form to be utilized only in a receiver equipped with a decodingdevice controlled in accordance with the coding schedule employed at thetransmitter. More particularly, the invention relates to a novelapparatus for use in such a secrecy communication system to preventtransient distortion attributable to transmission band width limitationsand to the coding or decoding operation. The novel arrangement of thepresent invention is particularly attractive when incorporated into theaudio encoding portion of a subscription television system and for thatreason is described in such an environment.

The invention may be practiced in either a transmitter or receiver andthus the term encoding is used herein in its generic sense to encompasseither coding at the transmitter or decoding at the receiver.

Numerous secrecy systems have been proposed in which an intelligencesignal, for example an audio signal, is coded by altering somecharacteristic of that signal, such as phase, usually at randomly spacedtime intervals determined by a coding schedule which is made known onlyto authorized receivers. Most of these secrecy systems do efiiectadequate coding or scrambling of the intelligence signal. However, adistortion may result from such a coding process that may be charged,inter alia, to the limited band width allotted to audio transmissionwhich prevents the translation of the entire range of frequencycomponents representing the extremely sharp amplitude excursions of thecoded audio signal produced by the .phase inversions in the codingoperation; almost an infinite band width would actually be required toduplicate such abrupt changes. Consequently, it is difficult ineffecting compensating phase inversions in the decoding process at thereceiver to avoid transient distortion pulses. Moreover, thecompensating phase changes in the decoding apparatus at the receiver .donot always occur in exact time coincidence with the corresponding phasechanges at the transmitter. Additional undesirable transient pulses areconsequently generated and reflected as transient distortion in thedecoded audio signal.

Such transient distortion, of course, detracts from thefidelity orquality of reproduction of the decoded intelligence signal, and sincethe distortion occurs essentially at the instants of phase inversion inthe decoding process, it may be eifectively removed by interposing asampling circuit and a low-pass filter in the audio channel inaccordance with the teachings of copending application Serial No.397,176, filed December 9, 1953, in the name of Howard K. Van Jepmond,and assigned to the present assignee. The operation of the sampler maybe so phased with respect to the code schedule that the decoded signalis only sampled (by means of a super-audible, periodically recurringsampling signal) at times other than the instants of phase inversion. Inthis way, the decoded or unscrambled signal is sampled range butsubstantially beyond that range.

or examined only during those intervals when no distortion transientsare present. The sampledl signal is then shaped in a low-pass filter,producing a distortion-free simulation of the original uncoded signal.

Even though the sampling arrangement of the copending Van Jepmondapplication does eliminate the distortion attributable to the band widthlimitations, it is nevertheless essential to provide audio translatingstages in the receiver that reproduce a relatively wide range offrequencies in order that the phase inversions of the received codedaudio signal are as sharp or as steep as possible. Otherwise, if thephase .inversions are stretched out or prolonged for a time durationgreater than the time separation of the sampling instants, thedistortion will obviously not be removed.

Thus, itis necessary to employ relatively wide band receiver audiostages in the Van Jepmond arrangement which respond to frequencies notonly in the audible As an incident to such wide band reception, it ispossible that unwanted super-audible or supersonic high frequency noisecomponents may also be accepted, particularly in a weak signal area.Moreover, whenthe sound translating stages are incorporated in atelevision receiver of the intercarrier type, low frequency video orpicture signal components that may be present in the intercarrier soundsignal will likewise be translated through the wide band audio stages.These noise and video components (both of which may be called extraneousnoise components here inasmuch as they are not part of the audiosignal), being outside of the audible range, would not ordinarilyintroduce any audible distortion, but since the sampling function in thereceiver is actually a modulation function, the undesired extraneousnoise components are beat or heterodyned down by the sampling signalinto the audible range. Consequently, the objectionable noise componentsmay be heard in the decoded audio signal.

The present invention stems from the previous Work of Van Jepmond andprovides a secrecy communication system which produces an encodedintelligence signal that is not only free of transient distortion but isalso substantially devoid of the above described undesired noise signalcomponents.

It is, accordingly, an object of the present invention to provide animproved secrecy communication system of the type disclosed in the VanJepmond application.

It is another object of the invention to provide a secrecy communicationtransmitter or receiver wherein the coded or decoded intelligence signalis substantially free of transient distortion and extraneous noisecomponents.

'It is still another object of the invention to provide an audioencoding arrangement for a subscription television system for producinga distortion-free and noisefree audio signal.

A secrecy communication system, constructed in accordance With thepresent invention, comprises a source of intelligence signal andencoding apparatus coupled to the source. The encoding apparatusincludes a control mechanism for developing an encoding signal havingcharacteristic variations representing a predetermined code schedule andan encoding device for utilizing the encoding signal to vary acharacteristic of the intelligence signal between a plurality ofdifferent modes in accordance with the code schedule to develop anencoded intelligence signal with the transitions during themode-changing intervals subject to introducing undesired distortion inthe encoded intelligence signal.

Means is coupled to the encoding apparatus and responds to the encodingsignal for developing a gating signal. Finally, the secrecycommunication system com-.

prises gating means coupled to the gating-signal-developing means and tothe encoding apparatus for utilizing the gating signal to effectivelydelete those portions of the encodedintelligence signal occurring duringthe modechan'ging intervals, thereby removing the undesired distortion.

The features of this invention-which are believed to be new are setforth with particularity in the appended claims. The invention, togetherwith further objects and advantages thereof, may best be understood,however, by reference to the following description in conjunction withthe accompanying drawings, inwhich: 4

Figure 1 is a schematic representation of a secrecy communicationsystem, specifically a subscription television receiver, constructed inaccordance with one embodiment of the invention; a

Figure 2 comprises a family of curves which appear at various points inthe receiver of Figure 1;

Figure 3 represents a portion of the receiver illustrated in Figure 1constituting another embodiment of the invention; and,

Figure 4 illustrates various'wave forms which are helpful in explainingthe operation of the embodiment of Figure 3. 1

While the present invention is applicable to any type of secrecycommunication and, moreover, to any type of coding, it isillustrated inconnection with a subscription television receiver for convenience.Furthermore, it should be understood that the arrangement of the presentinvention may be applied to the transmitter as well as the receiverportion of a secrecy system since it is often desirable to eliminate anytelltale transient pulses that may be introduced into the audio signalby the coding process each time a mode change is made, and which may besubsequently derived from the audio signal and utilized in unauthorizeddecoding.

, The receiver illustrated in Figure 1 is constructed to utilize atelecast originating at a transmitter which is made in accordance withcopending application Serial No.,366,727, filed July 8, 1953, and issuedSeptember 16, 1958, as Patent No. 2,852,598, in the name of Erwin M.Roschke, and assigned to the present assignee. Briefly, in :thatapplication a counting device responds to line-synchronizing pulses todevelop a square wave coding signal having amplitude changes occurringduring the line-retrace interval following each succession of 15line-trace intervals. During the field-retrace intervals, coding pulsesare developed and supplied to various input circuits of a bi-stablemultivibrator to eifect actuation thereof, preferably in random fashion.The counting device is re-phased during each field-retrace intervalunder the control of the bi-stable multivibrator and thus the squarewave coding signal from the counter is phase modulated in a randommanner. The phase modulator coding signal is employed to efliect modechanges in the transmitter by alternately introducing and then removinga time delay of the video with respect to the synchronizing components.The code signal pulses may be transmitted'along with the video signalduring the field-retrace intervals to facilitate the proper phasing of asimilar square wave signal at the receiver.

The coding signal is also used in the Roschke system to encode the audiointelligence. This is accomplished by applying it to the deflectionelectrodes of a beamdefiection device having a control grid, which ismodulated in accordance with the uncoded audio intelligence, and furtherhaving a pair of collector anodes connected to opposite terminals of theprimary winding of an output transformer. With this arrangement, thephase of the audio signal is effectively inverted at the secondarywinding of the transformer each time the beam switches from one anode tothe other, and this occurs each time there is an amplitude variation ofthe square wave coding signal.

The audiophase inversion process of the Roschlre system is subject tothe introduction of transient distortion since the mode changes may notoccur in exact synchronism at the transmitter and receiver, and thelimited band width transmission precludes the reproduction of extremelysharp phase inversions. Consequently, the receiver of Figure l issimilar to that shown in Roschke but is further adapted to prevent theintroduction of any noise or transient components in the decoded audiosignal.

More specifically, the receiver comprises a radio-frequency amplifier 10having input terminals connected to an antenna circuit 11 and outputterminals'connected to a first detector 12. This detector is coupledthrough an intermediate-frequency amplifier 13 to a seconddetector 14which, in turn, is connected to the input circuit of a video amplifier15. The output circuit of the video amplifier is connected through avideo decoder 16 to the input electrodes of a cathode-rayimage-reproducing device 19.

Decoder 16 may be similar to that disclosed and claimed in copendingapplication Serial No. 243,039, filed August 22, 1951, and issued August7, 1956, as Patent 2,758,153, in the name of Robert Adler, and assignedto the present assignee. It may comprise a beam-deflection tube having apair of output circuits which may be selectively coupled into the videochannel as the electron beam of the tube is deflected from one to theother of two collector anodes in synchronism with mode changes of thetransmitted signal. As mentioned hereinbefore, these mode changes. takethe form of variations in' the timing of the videocomponents relative tothe synchronizing components of the received composite televisionsignal. Consequently, the output circuit coupled to one anode segmentincludes a time delay network While the output connected to the otheranode segment does not, and the timing variations are compensated,efifectively to decode the television signal, as the beam of thedeflection tube is switched between its anodes. This switching effect isaccomplished by means of a deflection-control or actuating signalapplied to video decoder 16, as ex: plained hereinafter. Second detector14 is also coupled to a synchronizingsignal separator 22 which iscoupled, in turn, to afieldsweep system 23 and to a line-sweep system24L The output terminals of sweep systems 23 and 24 are connectedrespectively to fieldand line-deflection elements (not shown) associatedwith image reproducer 19.

Video amplifier 15 is also connected to an-amplifier and amplitudelimiter 26 which, in turn, is coupledto a discriminator detector 27. Theoutput terminals'of detector 27 are connected through a delay line 28 toone pair of input terminals of an encoding device in the form of anaudio decoder 30. This decoder, as explained briefly hereinbefore and indetail in the aforementioned Roschke application, may comprise abeam-deflection device which is actuated in accordance with the codingschedule of the telecast to efiect compensating phase inversions of thecoded audio signal to effectively decode that signal.

Alternatively, audio decoder 30 may comprise a phase splitter and anelectronic selector switch as shown in copending application Serial No.513,757, filed June 7, 1955, in the name of Walter S. Druz, and assignedto the present assignee. In that application, the phase splittersupplies the coded audio signal to the electronic selector switch inpush-pull relationship, namely, with two different phases apart. Theswitch is actuated in accordance with the coding schedule to selectcertain portions of each of the two signals from the phase splitter. Thedecoded audio signal developed in decoder 30 contains transientdistortion during the mode-changing intervals and this is removed, inaccordance with the invention, by connecting the output of decoder 30 toa normally-open gate circuit 32 through a resistor 33. Gate 32 is of abridge type and includes a unidirectional device, such as a diode 34, ineach ofits four arms or legs -directional potential such as a battery41.

amass fine corner Act the bridge is connected to resistor 33 .and thediagonally opposite corner C is connected to one side of a charging orstorage condenser 35, the other side of which is connected to ground.The other two diagonally opposite corners, B and D, are connectedtogether through a series arrangement of a resistor 37, the secondarywinding 38 of a transformer 40 and a source of uni- The ungroundedterminal of condenser 35 is connected through an audio amplifier 44 tothe input terminals of a speaker 45.

Gate circuit 32 is so arranged that source 41 renders all of the diodes34 normally conductive, and therefore an A.C. signal applied betweencorner A and ground :will be translated through the bridge to corner Cand thus impressed across condenser 35. the gate is shut off or closedduring certain time intervals will be discussed hereinafter.

In order to produce a deflection-control signal for The manner in whichvideo decoder 16 and audio decoder 30, a control mechanism or decodingsignal source 48 is connected directly to video decoder 16 and to audiodecoder 30 through a delay line 49. Decoding signal source 48 provides asquare Wave encoding (decoding) signal, exhibiting amplitude variationsduring line-retrace intervals representing the code schedule of thetelecast, to decoders 16 and 30 which is identical to that supplied tothe corresponding circuits at the transmitter of the aforementionedRoschke application, Serial No. 366,727. Of course, the manner in whichthe coding signal at the transmitter and the corresponding decodingsignal at the receiver are developed is entirely immaterial to thepresent invention; for that reason, source 48 has been shown merely asone block in the drawing for the sake of simplicity. For example, inaccordance with the Roschke application, the square wave decoding signaldeveloped incontrol mechanism 48 may be synchronized and phased withrelation to the counterpart coding square wave at the transmitter bymeans of signal bursts transmitted along With the television signalduring vertical-retrace intervals. The

.phase modulated square 'wave from source 48 eifects operation of audiodecoder 30 during line-retrace inter- ,vals in order to realizecompensating phase inversions of .the coded audio during such intervals.

In order to develop a gating signal for controlling the condition ofgate 32, in accordance with the present invention, two differentiatorsand rectifiers 51, 52 are connected to the output of decoding signalsource 48. Unit 51 is connected through a phase inverter 54 to one pairof input terminals of an adder 53, and dilferentiator and rectifier 52is connected directly to another pair of input terminals of the adder.The output of adder 53 is connected to the input circuit of amono-stable multivibrator 55 which is connected in turn to the primaryWinding 56 of transformer 40 in normally-open gate 32. With thisarrangement, gating pulses are applied to primary winding 56 with propermagnitude and polarity to render diodes 34 non-conductive during theoccurrence of such pulses. Consequently, gate 32 is closed at thoseinstants.

In the operation of the described receiver of Figure 1, the codedtelevision signal is intercepted by antenna 11, amplified inradio-frequency amplifier 10 and heterodyned to the selectedintermediate frequency of the receiver in first detector 12. Theresulting intermediate-frequency signal is amplified inintermediate-frequency amplifier 13 and detected in second detector 14to produce a composite video signal. This latter signal is amplified invideo amplifier 15, translated through video decoder 16 and impressed onthe input electrodes of image reproducer 19 to control the intensity ofthe cathode-ray beam of the reproducing device in well known manner.Video decoder 16 receives a decoding signal from decoding signal source48 which has amplitude variations occurring in exact time coincidencewith amplitude variations of the coding signal applied as adeflection-control signal 6 to a corresponding video coder in thetransmitter of the aforementioned Roschke application, Serial No.366,727, so that the video components applied to the input electrodes ofimage reproducer 19 are suitably compensated or decoded to elfectintelligible image reproduction.

The synchronizing components of the received signal are separated inseparator 22, the field-synchronizing components being utilized tosynchronize sweep system 23 and, therefore, the field scansion of theimage reproducer, while the line-synchronizing pulses are utilized tosynchronize sweep system 24 and, therefore, the line scansion of device19.

Consideration will now be given to the particular manner in whichtransient distortion that may otherwise result from the transmissionband width limitations and the audio decoding process is eliminated inaccordance with the invention, with reference to the idealized signalwave forms of Figure 2 which appear at certain points within the audiosection of the receiver, which points are indicated by encircledreference letters corresponding to the designations of the curves inFigure 2. An intercarrier sound signal derived from video amplifier 15is amplified and amplitude limited in unit 26 and detected indiscriminator detector 27 to develop the coded audio signal of curve E.v

The signal is illustrated for convenience :as a sinusoidal signal wavehaving a frequency of approximately 800 cycles per second andcharacterized by various phase inversions 60 occurring in a patternestablished by the audio coder at the transmitter. Since mode changesare made in the transmitter of the Roschke application after every 15line-trace intervals, phase inversions 60, which take place at the modechanges, occur at a frequency of approximately 1,000 cycles per secondunder the present United States standards. With the exceptionof thethird phase inversion 60 from the left in curve B, which just happens tooccur at a zero cross-over point of the sinusoidal signal, the phaseinversions do not occur instantaneously with an infinite slope butrequire a finite time interval as shown by the slanting rather thanvertical configuration of the wave forms at those mode-changingintervals. As mentioned hereinbefore, even if the phase inversions occurinstantaneously at the transmitter, it Would require the transmission ofan infinitely wide band Width to transmit all the frequency componentswhich represent such instantaneously abrupt phase inversions.

The coded audio signal of curve B is delayed in delay line 28 to formthe signal of curve B for application to audio decoder 30. Thecharacteristics of delay line 28 are adjusted so that it introduces atime delay greater than the time duration of a phase inversion 60, for

reasons which will become apparent later.

In order to effect compensating. phase inversions and accomplishdecoding of the coded audio signal of curve E, decoding signal source orcontrol mechanism 48 develops the encoding signal of curve F havingamplitude variations occurring in time coincidence with the mode changesintroduced at the transmitter and, consequently, in synchronism with thephase reversals of the received coded audio signal of curve E. Curve Fis translated through delay line 49 to develop the signal of curve F forapplication to decoder 30. Delay line 49 introduces exactly the samedelay to the signal of curve F as that introduced to Waveform E by delayline 28 in order to compensate for the delay of network 28.

Encoding signal F eifects a phase reversal of the coded audio signal ofcurve E in decoder 30 in response to each amplitude variation of curve Fto produce at the output terminals of the decoder a decoded audio signalhaving the wave shape shown in curve G. From an examination of thesignal of curve G it will be seen that there is a spike or pie cut inthe sinusoidal signal at each mode-changing interval, with the exceptionof the third one from the left, due to the prolonged rather thaninstantaneous phase changes in the coded signal decreases in amplitude.

audio signal of curve E. Additionally, undesirable switch- .ingtransients, shown as pulses 61 in curve G, may be introduced during the.decoding process. ifdecoder 30 takes the form of a beam-deflection tube,

For example,

greater'in amplitude than the audio intelligence signal.

It is apparent that the distortion of the sinusoidal signal of curve G,if not eliminated, would detract from the listening quality ratherconsiderably. In accordance with'the present invention, the"'dis'tortion is deleted by applying the square wave'decoding signal ofcurve F to differentiator and rectifier units 51 and 52. Thesedifierentiators and rectifiers differentiate both the positive andnegative amplitude excursions of the signal of curve F but unit 51 onlypasses the negative differentiated pulses to form the signalof curve Hand unit 52 only passes the positive differentiated pulses to form thesignal of curve I. The pulses of curve H are inverted in phase inverter54 to form the positive pulses of curve K, and the signals of bothcurves J and K are added in adder 53 to form the signal of curve L.Thus, a rather sharply defined pulse is produced for each mode change ofthe coded audio of curve E. The pulses of curve L are then applied tomono-stable multivibrator 55 to produce the elongated pulses of curve MThe parameters of the multivibrator are so adjusted that once it istriggered from its normal to its'abnormal condition, it will remainthere for a time interval slightly exceeding the duration assignedmode-changing interval, may be employed as gating pulses to close gate32 and therefore to delete or remove the distortion-occurring during themode-changing intervals in curve G.

Turning now to the operation of normally-open gate circuit 32, duringthe time intervals when a pulse of curve M is not applied to the gatethe decoded audio signal of curve G is translated directly through thegate to the input of audio amplifier 44 without alteration. Thepotential on storage condenser 35 follows the applied decoded audiosignal of curve G, charging through gate 32 when the decoded audioincreases in amplitude and discharging also through the gate when thedecoded audio However, during the intervals of the gating pulses ofcurve M, negative pulses are developed in secondary winding 38 oftransformer 40 to render each of the diodes 34 non-conductive. Thecircuit from audio decoder 30 to storage condenser 35 is effectivelyinterrupted during the pulses of curve M and thus the condenser, beingisolated, cannot follow the instantaneous amplitude changes of thedecoded audio. Since the charging and discharging circuit for condenser35 is open during those intervals, the condenser will hold the chargewhich it assumed immediately prior to each mode changing intervalthroughout the duration of the pulses of curve M. Upon the terminationof each gating pulse gate 32 returns to its normally-open condition,completing the charging and discharging circuit for condenser 35, andthe decoded audio signal is once again impressed across the condenser,which thereupon changes its charge level to assume the new instantaneouspotential of the applied signal, as shown by the signal of curve N whichis the signal applied to amplifier 44. From a close study of the waveform of curve N it may be observed that the decoded audio signal remainsat its instantaneous value immediately preceding each gating pulse ofcurve M for the duration of each gating pulse, at which time itchanges-in exponential fashion to the value ofthe applied decoded audio.

44 and is applied to speaker 45. Even though the wave form of curve Ndoes contain some discontinuities from a true sinusoidal shape, it isquite acceptible and constitutes a considerable improvement over thesignal of curve G. Actually, the discontinuities of the'signal of curveN are smoothed out somewhat due to the fact that amplifier 44 andspeaker 45 have a limited response range and those discontinuities arerepresented by high frequency components. 1

Inasmuch as the audio section of the subscription television receiverofFigure 1 responds to a frequency modulated signal, condenser 35 andresistor 33 may also serve as the required de-emphasis network. e

The reason'for' including delay line 28 in the' audio channel should nowbe apparent. By delaying the audio signal, the amplitude excursions ofthe undelayed decoding signal of curve F may be utilized to develop theactuating pulses of curve M, each of which pulses individually embracesthe distortion components during an assigned one of the mode-changingintervals in the decoded audio of curve G. In this way, it is possibleto anticipate each mode change and then delete or remove the portions ofthe decoded audio during the modechanging intervals.

It should also be now apparent that undesirable noise components are notbeat down into the audible range, as may occur with the Van Iepmondarrangement, since a sampling signal exhibiting a super-audiblefrequency is not employed. 1 1

By way of summary, amplifier and limiter 26 and detector 27 constitute asource of intelligencesignal, namely, a source of a coded audio curve,curve B. Encoding apparatus is coupled to this source through delay line28 and includes a control mechanism, such as decoding signal source 48,for developing an encoding signal (curve F) having characteristic(amplitude) variations representing a predetermined code schedule and anencoding device (audio decoder 30) for utilizing the encoding signal tovary a characteristic (phase) of the intelligence signal between aplurality of different modes in accordance with the code schedule. Anencoded intelligence signal (curve G) is therefore developed with thetransitions during the mode-changing intervals subject to introducingundesired distortion. Difierentiator and rectifier units 51 and 52,phase inverter 54, adder 53 and mulivibrator 55 constitute means coupledto the encoding apparatus, namely, to control mechanism 48, andresponsive to the encoding signal (curve F) for developing a gatingsignal (curve M). This gating signal consists of a series of pulses eachof which corresponds to an assigned mode-changing interval. Finally, thesecrecy communication system includes gating means in the form ofnormally-open gate circuit 32 which is coupled to thegating-signa-l-developing means (namely, to the output of multivibrator55) and to the encoding apparatus (to audio decoder 30) for utilizingthe gating signal to effectively delete those portions of the encodedintelligence signal occurring during the mode changing intervals (toproduce the signal of curve N), thereby removing the undesireddistortion.

In discussing the operation of gate circuit 32, it'was pointed out thatcondenser 35 maintained a constant potential during each gating pulse ofcurve M and at the termination of each pulse the charge varied inexponential manner to assume the instantaneous potential of the applieddecoded audio signal. A small portion of the Wave form of curve N,including the first complete sinusoidal cycle, has been redrawn inFigure 4 man expanded time scale and with increased amplitude toillustra-te the manner in which the charge on condenser 35 changesexponentially rather than instantaneously upon the termination of thefirst gating pulse of curve M, Thedashed line in the portion of curve Nshown in Figure 4 illustrates the desired wave form of the signal 9during the mode-changing interval and immediately following. The areabetween the dashed line and the actual 'Wave shape represents thedistortion of the signal as applied to audio amplifier 44. As mentionedhereinbefore, that wave shape is smoothed out because of the responsecharacteristics of amplifier 44 and speaker 45 so the slight distortionin the signal is not objectionable.

However, to improve the arrangement of Figure 1 and therefore todecrease further the amount of distortion remaining in the decoded audiosignal before application to audio amplifier 44, it is desirable todecrease the area between the solid and dashed lines in curve N ofFigure 4. In the embodiment of Figure 3, such distortion is reduced byconnecting decoder 30 directly to normallyopen gate circuit 32, therebyeliminating the resistance of resistor 33, and by decreasing thecapacitance of condenser 35. In this way, condenser 35 charges anddischarges instantaneously as shown by the signal of wave form P, whichis developed in the output of gate 32 in Figure 3 in response to thefirst complete sine wave cycle of curve G. Upon the termination of thegating pulse, condenser 35 immediately acquires the potential of theapplied audio. The area between the dashed and solid line curves of waveform P is consequently considerably less than that of wave form N,resulting in less distortion in the signal. The signal of curve P isthen applied to a separate de-emphasis circuit 63 through a buffer 64wherein the high frequency components are eiiectively removed due to thecharacteristics of the de-emphasis network to produce the signal ofcurve Q for application to amplifier 44. It will be apparent from acomparison of the Wave shapes of curves N and Q that the signal appliedto amplifier 44 in Figure 3 has considerably less distortion than thatapplied to amplifier 44 in Figure l. Amplifier 44 in Figure 3 furthersmoothes out the wave shape of curve Q so that the signal to whichspeaker 45 responds is substantially a true simulation of the originalsinusoidal signal originating at the transmitter.

The invention provides, therefore, an improved secrecy communicationsystem for producing an encoded intelligence signal that is relativelyfree of transient distortion which may arise due to transmission bandwidth limitations and/or may be introduced during the coding or decodingprocess. This is very effectively achieved by anticipating each modechange and then deleting those portions of the encoded intelligencesignal during the mode'changing intervals.

While particular embodiments of the invention have been shown anddescribed, modifications may be made, and it is intended in the appendedclaims to cover all such modifications as may fall within the truespirit and scope of the invention.

I claim:

1. A secrecy communication system comprising: a source of intelligencesignal; encoding apparatus coupled to said source including a controlmechanism for developing an encoding signal having characteristicvariations representing a predetermined code schedule and an encodingdevice for utilizing said encoding signal to vary a characteristic ofsaid intelligence signal between a plurality of difierent modes inaccordance with said code schedule to develop an encoded intelligencesignal With the transitions during the mode-changing intervals subjectto introducing undesired distortion in the encoded intelligence signal;means coupled to said encoding apparatus and responsive to said encodingsignal for developing a gating signal; and gating means coupled to saidgating-signal-developing means and to said encoding apparatus forutilizing said gating signal to effectively delete those portions ofsaid encoded intelligence signal occurring during the mode-changingintervals, thereby removing said undesired distortion.

2. A secrecy communication system comprising: a source of intelligencesignal; encoding apparatus coupled to said source for varying acharacteristic of said intelligence signal between a plurality of difierenfmodes in accordance with a predetermined code schedule to developan encoded intelligence signal with the transitions during themode-changing intervals subject to introducing undesired distortion inthe encoded intelligence signal; means coupled to said encodingapparatus for developing a gating signal consisting of a series ofpulses each of which corresponds to an assigned mode-changing interval;and gating means coupled to said gating-signal-developing means and tosaid encoding apparatus for utilizing said gating signal to effectivelydelete only those portions of said encoded intelligence signal occurringduring the modechanging intervals, thereby removing said undesireddistortion.

3. A secrecy communication system comprising: an audio signal source;encoding apparatus coupled to said source including a control mechanismfor developing an encoding signal having characteristic variationsrepresenting a predetermined code schedule and a phase-invertingencoding device for utilizing said encoding signal to invert the phaseof said audio signal at selected times determined by said code scheduleto develop an encoded audio signal with the transitions at said selectedtimes subject to introducing undesired distortion in the encoded audiosignal; means coupled to said encoding apparatus and responsive to saidencoding signal for developing a gating signal; and gating means coupledto said gatingsignal-developing means and to said encoding apparatus forutilizing said gating signal to efiectively delete those portions ofsaid encoded audio signal occurring at said selected times, therebyremoving said undesired distortion.

4. A secrecy communication system comprising: an audio signal source;encoding apparatus coupled to said source including a control mechanismfor developing an encoding signal having amplitude variationsrepresenting a predetermined code schedule and a phase-invertingencoding device for utilizing said encoding signal to vary the phase ofsaid audio signal between a plurality of different modes in accordancewith said code schedule to develop an encoded audio signal with thetransitions during the mode-changing intervals subject to introducingundesired distortion in the encoded audio signal; a utilizing circuit; agate circuit coupling said encoding apparatus to said utilizing circuitand normally conditioned to translate said encoded audio signal to saidutilizing circuit; means coupled to said control mechanism andresponsive to said encoding signal for developing a gating signalconsisting of a series of pulses each of which corresponds to anassigned mode-changing interval; and means coupling saidgating-signal-developing means to said gate circuit to render said gatecircuit ineffective during each mode changing interval, eifectively todelete those portions of said encoded audio signal occurring during themodechanging intervals to thereby remove said undesired dis.- tortion.

5. An audio decoding arrangement for a subscription television receivercomprising: a source of coded audio signal having a number of phaseinversions occurring in accordance with a predetermined code schedule; adecoding apparatus coupled to said audio signal source for reinvertingthe phase of said coded audio signal at selected times determined bysaid predetermined code schedule to develop a decoded audio signal withthe transitions during the selected times subject to introducingundesired distortion in the decoded audio signal; means coupled to saiddecoding apparatus for developing a gating signal consisting of a seriesof pulses each of which corresponds to an assigned one of said selectedtimes; and gating means coupled to said gating-signal-developing meansand to said decoding apparatus for utilizing said gating signal toeifectively delete only those portions of said decoded audio signaloccurring during the selected times, thereby removing said undesireddistortion.

6. A subscription television system comprising: a

source of video signal; video encoding apparatus coupled to said videosignal source for varying the mode of translation of said video signalat mode-changing intervals audio signal; means coupled to said audioencoding apparatus for developing a gating signal consisting of a seriesof pulses each of which corresponds to an assigned -mode-changinginterval; and gating means coupled to said gating-signal-developingmeans and to said audio encoding apparatus for utilizing said gatingsignal to effectively delete only those portions of said encoded audiosignal occurring during the mode-changing intervals, thereby removingsaid undesired distortion. i

7. A secrecy communication system comprising: a source of intelligencesignal; encoding apparatus coupled to said source including a controlmechanism for developing an encoding signal having characteristicvariations representing a predetermined code schedule and an encodingdevice for utilizing said encoding signal to vary a characteristic ofsaid intelligence signal between a plurality of different modes inaccordance with said code schedule to develop an encoded intelligencesignal with the transitions during the mode-changing intervals subjectto introducing undesired distortion in the encoded intelligence signal;a gate circuit coupled to said encoding apparatus and including anormally-open charging-discharging circuit and a storage condenser towhich is translated said encoded intelligence signal through saidcharging-discharging circuit; a utilizing circuit coupled across saidstorage'condenser; means coupled to said control mechanism andresponsive to said encoding signal for developing a gating signalconsisting of a series of pulses each of which corresponds to anassigned mode-changing interval; and means system comprising: a sourceof audio signal; encoding apparatus coupled to said source including acontrol mechanism for developing an encoding signal havingcharacteristic variations representing a predetermined code schedule andan encoding device for utilizingsaid encoding signal to vary acharacteristic of said audio signal between a plurality of differentmodes in accordance with said code schedule to develop an encoded audiosignal with the transitions during the mode changing intervals subjectto introducing undesired distortion in the encoded audio signal; a gatecircuit coupled to said encoding apparatus and including a normally-opencharging-discharging circuit and a storage condenser to which istranslated said encoded audio signal through said charging-dischargingcircuit, said condenser and chargingdischarging circuit forming ade-emphasis network; a utilizing circuit coupled across said storagecondenser; means coupled to said control mechanism and responsive tosaid encoding signal for developing a gating signal consisting of aseries of pulses each of which corresponds to an assigned mode changinginterval; and means coupling said gating-signal-developing means to saidgating circuit to close said charging-discharging circuit during eachmode-changing interval in orderto maintain said storage. condenser atthecharge towhich it assumed immediately preceding the mode-changinginterval, e fiectively to delete those portions of said encoded audiosignal occurring during the mode-changing intervals and to therebyremove said undesired distortion.

9. A secrecy communication system comprlsingi a source of intelligencesignal; encoding apparatus coupled -to said source including a controlmechanism for developing an encoding signal having characteristicvariations representing a predetermined code schedule and an encodingdevice for utilizing said encoding signal to vary a characteristic ofsaid intelligence signal between a plurality of difierent modes inaccordance with said code schedule to develop an encoded intelligencesignal with the transitions during the mode-changing intervals subjectto introducing undesired distortion in the encoded intelligence signal;means coupled to said encoding apparatus and'responsive to said encodingsignal for developing a gating signal; gating means coupled to said'gatingsignal-developing means and .to said encoding apparatus forutilizing said gating signal to efiectively delete those portions ofsaid encoded intelligence signal occurring during the mode-changingintervals, thereby removing said undesired distortion but introducing adiscontinuity in said encoded intelligence signal during eachmodechanging interval represented by relatively high frequencycomponents; and frequency-selective means coupled to said gating meansfor removing substantially all of said high frequency componentseffectively to smooth out the discontinuities in said encodedintelligence signal.

10. A frequency-modulation secrecy communication system comprising: asource of audio signal; encoding apparatus coupled to said sourceincluding a control mechanism for developing anencoding signal havingcharacteristic variations representing 'a predetermined code scheduleand an encoding device for utilizing said encoding signal to vary acharacteristic of said audio signal between a plurality of diiferentmodes in accordance withsaid code schedule to develop an encoded audiosignal with the transitions during the mode-changing intervals subjectto introducing undesired distortion in the encoded audio signal; meanscoupled to said encoding apparatus and responsive to said encodingsignal for developing a gating signal; gating means coupled to saidgating-signal-developing means and to said encoded apparatus forutilizing said gating signal to cflectively delete those portions ofsaid encoded audio signal occurring during the mode-changing intervals,thereby removing said undesired distortion but introducing adiscontinuity in said encoded audio signal during ,each modechanginginterval represented by relatively high frequency components; andfrequency selective means including a de-emphasis network coupled tosaid gating means for removing substantially all of said high frequencycomponents effectively to smooth out the discontinuities insaid encodedintelligence signal.

11. A secrecy communication receiver comprising: a source of codedintelligence signal having a characteristic varying between a pluralityof difierent modes ,during mode-changing intervals in accordance with apredetermined code schedule; a control mechanism for developing adecoding signal having characteristic variations representing said codeschedule and occurring during said mode-changing intervals; delay means;a decoding device coupled to said source and said control mechanismthrough said delay; means to develop a decoded'intelligence signal withthe transitions during the mode-changing intervals subject tointroducing undesired distortion in the decoded intelligence signal;means coupled to said control mechanism and responsive to said decodingsignal for developing a gating signal which effectively anticipates eachmode change of said decoded intelligence signal; and gating meanscoupled to 'said gating-signal-developing means and to said decodingdevice for utilizing said gating signal to effectively delete thoseportions of said decoded intelligence signal occurring duringthemode-changing intervals, thereby removing said undesired distortion.

12. A secrecy communication receiver comprising: a source of coded audiosignal having a number of phase inversions occurring duringmode-changing intervals in accordance with a predetermined codeschedule; a control mechanism for developing a decoding signal havingamplitude variations representing said code schedule and occurringduring said mode-changing intervals; a first delay device forintroducing a predetermined time delay to an applied signal; a seconddelay device for introducing said same predetermined time delay to anapplied signal; a decoding device coupled to said source through saidfirst delay device and to said control mechanism through said seconddelay device for reinverting the phase of said coded audio signal duringsaid mode-changing intervals to develop a decoded audio signal with thetransitions during the mode-changing intervals subject to introducingundesired distortion in the decoded audio signal; means coupled to saidcontrol mechanism for developing a gating signal consisting of a seriesof pulses each of which corresponds to an assigned mode-changinginterval and each of which embraces in point of time the undesireddistortion occurring during its assigned mode-changing interval; andgating means coupled to said gating-signal-developing means and to saiddecoding device for utilizing said gating signal to eifectively deletethose portions of said decoded audio signal occurring during themode-changing intewals, thereby removing said undesired distortion.

References Cited in the file of this patent UNITED STATES PATENTS2,656,411 Morris et al Oct. 20, 1953

